1. Field of the Invention
The present invention relates to a method for bonding semiconductor substrates and an apparatus therefor.
2. Description of the Related Art
Two semiconductor wafers, whose surfaces are mirror-ground, are placed opposite to each other and brought into contact with each other in a clean atmosphere. These semiconductor wafers are thus bonded together to form a substrate for forming a semiconductor element and the substrate is called a bonded wafer.
According to a method for bonding two wafers by bringing their mirror-ground surfaces into contact with each other, they can be bonded without using any adhesive and, therefore, a semiconductor element can be formed by subjecting the bonded wafers to the same high-temperature and chemical processings as those to which a single wafer is subjected.
Since, however, the wafers are bonded under the atmospheric pressure, sometimes air bubbles are left between the wafers.
A bonding apparatus which solves the problem of air bubbles formed by bonding the wafer under the atmospheric pressure is disclosed in U.S. Pat. No. 4,752,180. In this apparatus, one of the wafers is warped, making it difficult for air bubbles to be formed even in atmospheric pressure.
To warp one of the wafers, however, a support base must be formed having an elastic member. If the elastic member has ripples or waves, those ripples will affect the wafer. As illustrated in FIGS. 2A and 2B, slight waviness 112 occurs on one of two wafers constituting bonded wafer 110 and, thus, air bubbles 114 are formed between the wafers.